Aquinasfan said that public scandal was ONE reason not to engage in it. That in no way implied that the absense of scandal would make it morally acceptable.
Literally, yes. Practically, no. Chaste cohabitors represent .001% of the cohabiting population (my statistics). IOW, they're statistically insignificant.
But cohabitation is a public scandal. And that's reason enough not to engage in it.
3) Does it still count if its a relatively short period of time (day before marraige, one week after engagement)?
Yes, because cohabitation is a public scandal. One doesn't have to be a Christian to understand the importance of bad example.
Correct. The Good Lord wants us to remain chaste until holy matrimony. Anything else is a divergence of His plans to be avoided as much as possible.